A linear actuator disclosed in Japanese Patent Application Publication No. 2002-372118 (JP2002-372118A) and Japanese Patent No. 4251789 (JP4251789) includes a hollow rotary shaft 103, 203 that is rotatable along with rotational motion of a motor, a ball screw nut 115, 215 fixed to the rotary shaft 103, 203, and a ball screw 117, 217 threadably engaged with the ball screw nut 115, 215. Rotational motion of the rotary shaft 103, 203 causes the ball screw 117, 217 serving as an output shaft to make linear motion in the axial direction of the rotary shaft 103, 203. In each of the linear actuators, the ball screw nut 115, 215 is screwed to a locknut 113, 213 threadably engaged with a thread portion 121, 221 provided on an outer peripheral portion of the rotary shaft 103, 203 so that the ball screw nut 115, 215 is fixed to the rotary shaft 103, 203 (see FIGS. 3 and 4). In the linear actuator disclosed in Japanese Patent No. 4251789 shown in FIG. 4, in particular, a gap 244 is formed between a flange portion 215B of the ball screw nut 215 and the locknut 213 with an end 203A of the rotary shaft 203 contacting the ball screw nut 215, as a result of which a tightening force for screwing the ball screw nut 215 and the locknut 213 reinforces fixation between the ball screw nut 215 and the locknut 213.
In the linear actuators according to the related art, however, the locknut for fixing the ball screw nut to the rotary shaft is threadably fixed to the outer peripheral portion of the rotary shaft, and therefore backlash or center displacement (displacement between the centers of rotation of two members) tends to occur between the outer peripheral portion of the rotary shaft and the inner peripheral portion of the locknut. Therefore, if the assembly precision is not improved in the linear actuators according to the related art, displacement may occur between the center of rotation of the rotary shaft and the center of rotation of the ball screw, which may cause the center of rotation of the ball screw to be inclined with respect to the axis of the rotary shaft.
In the linear actuators according to the related art, the ball screw nut is fixed to the locknut using a plurality of screws with an end of the rotary shaft contacting the ball screw nut. Therefore, if the ball screw nut is fixed to the locknut using the plurality of screw members and accordingly screw tightening force is large, the end of the rotary shaft may be deformed or damaged.